U.S. patent number 7,223,244 [Application Number 10/849,066] was granted by the patent office on 2007-05-29 for system and method for monitoring hypercapnic ventilatory response.
This patent grant is currently assigned to Pacesetter, Inc.. Invention is credited to Steve Koh.
United States Patent |
7,223,244 |
Koh |
May 29, 2007 |
System and method for monitoring hypercapnic ventilatory
response
Abstract
An exemplary method includes determining a parameter related to
CO.sub.2 concentration in a patient's blood, as well as determining
a parameter related to respiration of the patient. The parameters
are then processed to diagnose a cardiac condition based at least
in part on the parameters.
Inventors: |
Koh; Steve (South Pasadena,
CA) |
Assignee: |
Pacesetter, Inc. (Sylmar,
CA)
|
Family
ID: |
38056724 |
Appl.
No.: |
10/849,066 |
Filed: |
May 18, 2004 |
Current U.S.
Class: |
600/532;
600/508 |
Current CPC
Class: |
A61B
5/145 (20130101); A61B 5/14539 (20130101); A61B
5/0535 (20130101); A61B 5/091 (20130101) |
Current International
Class: |
A61B
5/08 (20060101) |
Field of
Search: |
;607/17-18,20 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Layno; Carl
Claims
What is claimed is:
1. A method comprising: determining a parameter related to CO.sub.2
concentration in a patient's blood; determining a parameter related
to respiration of the patient; and comparing the parameter related
to CO.sub.2 concentration to the parameter related to respiration
to diagnose a physiologic condition based at least in part on the
comparison of the parameters.
2. The method of claim 1 wherein determining a parameter related to
CO.sub.2 comprises determining a pH level of the patient's
blood.
3. The method of claim 1, wherein determining a parameter related
to CO.sub.2 comprises determining the CO.sub.2 level of the
patient's blood.
4. The method of claim 1, wherein determining a parameter related
to respiration comprises determining a tidal volume value.
5. The method of claim 4, wherein determining a tidal volume value
comprises determining breath-by-breath tidal volume values.
6. The method of claim 1, wherein processing the parameters
comprises calculating a ratio of the parameters.
7. The method of claim 6, wherein calculating the ratio comprises
calculating a ratio of CO.sub.2 to tidal volume.
8. The method of claim 1, wherein processing the parameters
comprises diagnosing a heart failure condition.
9. The method of claim 1, wherein processing the parameters
comprises analyzing the parameters over a period of time to detect
a trend.
10. The method of claim 1, further comprising determining if the
patient is inactive, and if the patient is not inactive, inhibiting
the processing of the parameters.
11. The method of claim 1, and further comprising detecting a
progression of heart failure based on the processing of the
parameters, and changing an operating parameter in response to
detecting a progression of heart failure.
12. The method of claim 1, wherein processing the parameters to
diagnose a physiologic condition based at least in part on the
parameters comprises diagnosing one of a progression of heart
failure, pulmonary congestion, and an increased likelihood for
Cheyne-Stokes Respiration (CSR).
13. An implantable device comprising: means for determining a
parameter related to CO.sub.2 concentration in a patient's blood;
means for determining a parameter related to respiration of the
patient; and means for comparing the parameter related to CO.sub.2
concentration to the parameter related to respiration to diagnose a
physiologic condition based at least in part on the comparison of
the parameters.
14. The implantable device of claim 13 wherein the means for
determining a parameter related to CO.sub.2 comprises means for
determining a pH level of the patient's blood.
15. The implantable device of claim 13, wherein the means for
determining a parameter related to CO.sub.2 comprises means for
determining the CO.sub.2 level of the patient's blood.
16. The implantable device of claim 13, wherein the means for
determining a parameter related to respiration comprises means for
determining a tidal volume value.
17. The implantable device of claim 13, wherein the means for
processing the parameters comprises means for calculating a ratio
of the parameters.
18. The implantable device of claim 13, wherein the means for
processing the parameters comprises means for diagnosing a heart
failure condition.
19. The implantable device of claim 13, wherein the means for
determining a parameter related to respiration comprises means for
determining breath-by-breath tidal volume values.
20. The implantable device of claim 13, wherein the means for
processing the parameters comprises means for analyzing the
parameters over a period of time to detect a trend.
21. The implantable device of claim 13, further comprising means
for determining if the patient is inactive and means for inhibiting
the processing of the parameters if the patient is not
inactive.
22. The implantable device of claim 13, and further comprising
means detecting a progression of heart failure based on the
processing of the parameters, and means for changing an operating
parameter in response to detecting a progression of heart
failure.
23. An implantable cardiac system comprising: an implantable device
comprising circuitry; one or more sensors configured for implant
within the patient and in communication with the circuitry, wherein
the one or more sensors are operative to sense a parameter related
to a CO.sub.2 level of the patient's blood and a parameter related
to respiration of the patient; and wherein the circuitry is
operative to compare the parameter related to the CO.sub.2 level
with the parameter related to respiration of the patient, and to
detect a change in a physiologic condition based on the
comparison.
24. The implantable cardiac system of claim 23 wherein the one or
more sensors are operative to detect a pH level of the patient's
blood.
25. The implantable cardiac system of claim 23, wherein the one or
more sensors are operative to detect the CO.sub.2 level of the
patient's blood.
26. The implantable cardiac system of claim 23, wherein the one or
more sensors are operative to detect a minute ventilation
value.
27. The implantable cardiac system of claim 23, wherein the
circuitry is operative to calculate a ratio of the CO.sub.2 and
respiration parameters.
28. The implantable cardiac system of claim 23, wherein the
circuitry is operative to diagnose a heart failure condition based
on a comparison of the parameter related to the CO.sub.2 level with
the parameter related to respiration of the patient.
29. The implantable cardiac system of claim 23, wherein the one or
more sensors are operative to determine a tidal volume value.
30. The implantable cardiac system of claim 23, wherein the
circuitry is operative to analyze the parameters over a period of
time to detect a trend.
Description
TECHNICAL FIELD
The present invention generally relates to devices, systems and/or
methods for diagnosing a patient's cardiac condition, and/or for
providing cardiac pacing therapy. More particularly, various
exemplary methods and systems determine one or more parameters
related to patient breathing and use such information for
diagnostics and/or therapy.
BACKGROUND
Congestive heart failure (CHF) is a condition that is often
associated with a weakened heart that cannot pump enough blood to
body organs. For example, as pumping action is lost, blood may back
up into the heart and other areas of the body, including the liver,
gastrointestinal tract, extremities and/or lungs. Implantable
cardiac therapy devices are often used to overcome the deleterious
effects caused by CHF, and in some cases to reverse the negative
remodeling of the heart. Some implantable cardiac devices can also
be programmed to compensate for worsening stages of CHF. For
example, as CHF progresses, the myocardium weakens, which typically
results in an increased left ventricular volume, also referred to
as left ventricular dysfunction (LVD). To compensate for the
increase in volume, a clinician may periodically measure a
patient's left ventricular diameter, or another parameter
associated with cardiac geometry, and program the implanted cardiac
therapy device accordingly. This technique, however, requires
clinical intervention, which consumes time and resources.
Some patients suffer from both congestive heart failure (CHF) and
Cheyne-Stokes Respiration (CSR), which is defined as abnormal
respiration in which periods of shallow/apneic breathing and deep
breathing alternate (also known as periodic breathing). It has been
found in studies that patients who suffer from both CHF and CSR
tend to have larger left ventricular end-diastolic volumes (LVEDV),
namely the volume of the left ventricle immediately prior to
contraction of the left ventricle.
Lung and tissue gas stores of CO.sub.2 affect the rapidity of the
CO.sub.2 exchange process from breathing, and thus have a direct
influence on the respiratory control system damping. When the
CO.sub.2 stores are relatively large, fluctuations in ventilation
exert a smaller effect on alveolar and arterial PCO.sub.2 changes.
Thus these gas stores act like a low-pass filter, attenuating the
effect of rapid ventilatory fluctuations more than slow changes in
ventilation.
As is well known, increased filling pressures (end-diastolic volume
pressure) can lead to pulmonary vascular congestion and
consequently, a decrease in pulmonary gas volume. This reduction in
gas store will promote instability by elevating plant gain in the
lung-chemoreflexor control. This gain is similar to hypercapnic
ventilatory response slopes, which indicate the body's ability to
expel CO.sub.2 following a period of hypoventilation (abnormally
slow and shallow respiration), which results in hypercapnia (high
levels of CO.sub.2 in the blood). It has also been discovered that
hypercapnic ventilatory response among CHF patients with CSR is
about double that compared to normal patients or those who suffer
from obstructive sleep apnea.
What is needed is a reliable and convenient system and method that
automatically determines progression and/or regression of heart
failure, and that optionally can adjust patient therapy
accordingly. Further, what is needed is a system that detects the
rate at which CO.sub.2 is expelled, and which uses that rate to
detect progression of CHF, and/or to identify patients with CHF who
are also likely have CSR.
SUMMARY
An exemplary method includes determining a CO.sub.2-related value
and a respiration-related value (e.g., tidal volume (TV)). The
CO.sub.2 and respiration values are compared to each other (in one
embodiment, by calculating a ratio of CO.sub.2 to TV), which
provides an indication of a cardiac condition, such as a surrogate
for heart failure progression and/or regression. The determined
information may be used, for example, to warn the patient or a
physician regarding progression of the heart failure condition, or
to automatically adjust one or more operating parameters of the
implanted cardiac device. Other exemplary methods, devices and/or
systems are also disclosed.
In one embodiment, the comparison of CO.sub.2 and respiration
levels is in the form of the ratio of a CO.sub.2 level to a
respiration value, e.g. tidal volume. This ratio is a surrogate for
hypercapnic ventilatory response, which may be used to detect
various heart failure characteristics, such as LV dysfunction, LV
end-diastolic volume or pressure, and the like. The ratio may also
be used to detect pulmonary congestion, and to identify those
patients who are more likely to suffer from CSR.
The various exemplary methods, devices and/or systems described
herein, and equivalents thereof (e.g., structural and/or
functional), are suitable for use in a variety of pacing therapies
and other cardiac related therapies.
BRIEF DESCRIPTION OF THE DRAWINGS
Features and advantages of the described implementations can be
more readily understood by reference to the following description
taken in conjunction with the accompanying drawings.
FIG. 1 is a simplified diagram illustrating an implantable
stimulation device in electrical communication with at least three
leads implanted into a patient's heart for sensing electrical
activity and for delivering stimulation and/or shock therapy.
FIG. 2 is a functional block diagram of a multi-chamber implantable
stimulation device illustrating basic elements that are configured
to process sensed signals and respond accordingly.
FIG. 3 is a flow diagram of an exemplary method for determining a
surrogate for hypercapnic ventilatory response.
FIG. 4 is a flow diagram of an exemplary method for determining a
tidal volume value.
FIG. 5 is a detailed flow diagram of an exemplary method for
determining the hypercapnic ventilatory response.
FIG. 6 is a flow diagram of an exemplary method for monitoring the
hypercapnic ventilatory response to detect a change in a
physiologic condition.
DETAILED DESCRIPTION
The following description is not to be taken in a limiting sense,
but rather is made merely for the purpose of describing the general
principles of the illustrative embodiments. The scope of the
described implementations should be ascertained with reference to
the issued claims.
Exemplary Stimulation Device
The techniques described below are intended to be implemented in
connection with any implantable monitoring and/or stimulating
device that is configured for implant within a patient.
FIG. 1 shows an exemplary stimulation device 100 in electrical
communication with a patient's heart 102 by way of three leads 104,
106, 108, suitable for generating potential fields, sensing
potentials, and/or delivering stimulation and shock therapy. The
right atrial lead 104, as the name implies, is positioned in and/or
passes through a patient's right atrium. The right atrial lead 104
optionally generates a potential field (e.g., in combination with
another electrode), senses atrial cardiac signals or other signals
and/or provides right atrial chamber stimulation therapy. As shown
in FIG. 1, the stimulation device 100 is coupled to implantable
right atrial lead 104 having, for example, an atrial tip electrode
120, which typically is implanted in the patient's right atrial
appendage. The lead 104, as shown in FIG. 1, also includes an
atrial ring electrode 121. Of course, the lead 104 may have other
electrodes as well.
To generate a potential field; sense potentials, left atrial
cardiac signals, and/or left ventricular cardiac signals; and/or to
provide chamber pacing therapy, particularly on the left side of a
patient's heart, the stimulation device 100 is coupled to a
coronary sinus lead 106 designed for placement in the coronary
sinus and/or tributary veins of the coronary sinus. Thus, the
coronary sinus lead 106 is optionally suitable for positioning at
least one distal electrode adjacent to the left ventricle and/or
additional electrode(s) adjacent to the left atrium. In a normal
heart, tributary veins of the coronary sinus include, but may not
be limited to, the great cardiac vein, the left marginal vein, the
left posterior ventricular vein, the middle cardiac vein, and the
small cardiac vein. Electrodes are optionally positioned in or via
such tributary veins.
Accordingly, an exemplary coronary sinus lead 106 is optionally
designed to generate a potential field, sense potentials or signals
and/or to deliver pacing therapy. For example, therapy may include
left ventricular pacing therapy using at least a left ventricular
tip electrode 122, left atrial pacing therapy using at least a left
atrial ring electrode 124, and shocking therapy using at least a
left atrial coil electrode 126. For a complete description of a
coronary sinus lead, the reader is directed to U.S. Pat. No.
5,466,254, "Coronary Sinus Lead with Atrial Sensing Capability"
(Helland), which is incorporated herein by reference. The coronary
sinus lead 106 further optionally includes electrodes for
stimulation of autonomic nerves.
Stimulation device 100 is also shown in electrical communication
with the patient's heart 102 by way of an implantable right
ventricular lead 108 having, in this exemplary implementation, a
right ventricular tip electrode 128, a right ventricular ring
electrode 130, a right ventricular (RV) coil electrode 132, and an
SVC coil electrode 134. Typically, the right ventricular lead 108
is transvenously inserted into the heart 102 to place the right
ventricular tip electrode 128 in the right ventricular apex so that
the RV coil electrode 132 will be positioned in the right ventricle
and the SVC coil electrode 134 will be positioned in the superior
vena cava. Accordingly, the right ventricular lead 108 is capable
of generating potential fields, sensing potential and/or cardiac
signals, and delivering stimulation in the form of pacing and shock
therapy to the right ventricle. An exemplary right ventricular lead
may also include at least one electrode capable of stimulating an
autonomic nerve and/or anchoring the lead, such an electrode may be
positioned on the lead or a bifurcation or leg of the lead.
FIG. 2 shows an exemplary, simplified block diagram depicting
various components of stimulation device 100. The stimulation
device 100 can be capable of treating both fast and slow
arrhythmias with stimulation therapy, including cardioversion,
defibrillation, and pacing stimulation. While a particular
multi-chamber device is shown, it is to be appreciated and
understood that this is done for illustration purposes only. Thus,
the techniques and methods described below can be implemented in
connection with any suitably configured or configurable implantable
device that can generate potential fields and/or sense potentials.
Accordingly, one of skill in the art could readily duplicate,
eliminate, or disable the appropriate circuitry in any desired
combination to provide a device capable of generating potential
fields and/or sensing potentials, and optionally treating
appropriate chamber(s) with cardioversion, defibrillation, and/or
pacing stimulation.
Housing 200 for stimulation device 100 is often referred to as the
"can", "case" or "case electrode", and may be programmably selected
to act as the return electrode for all "unipolar" modes. Housing
200 may further be used as a return electrode alone or in
combination with one or more of the coil electrodes 126, 132 and
134 for shocking purposes. Housing 200 further includes a connector
(not shown) having a plurality of terminals 202, 204, 206, 208,
212, 214, 216, and 218 (shown schematically and, for convenience,
the names of the electrodes to which they are connected are shown
next to the terminals).
To achieve right atrial generating, sensing and/or pacing, the
connector includes at least a right atrial tip terminal (A.sub.R
TIP) 202 adapted for connection to the atrial tip electrode 120. To
achieve left chamber generating, sensing, pacing, and/or shocking,
the connector includes at least a left ventricular tip terminal
(V.sub.L TIP) 204, a left atrial ring terminal (A.sub.L RING) 206,
and a left atrial shocking terminal (A.sub.L COIL) 208, which are
adapted for connection to the left ventricular tip electrode 122,
the left atrial ring electrode 124, and the left atrial coil
electrode 126, respectively.
To support right chamber generating, sensing, pacing, and/or
shocking, the connector further includes a right ventricular tip
terminal (V.sub.R TIP) 212, a right ventricular ring terminal
(V.sub.R RING) 214, a right ventricular shocking terminal (RV COIL)
216, and a superior vena cava shocking terminal (SVC COIL) 218,
which are adapted for connection to the right ventricular tip
electrode 128, right ventricular ring electrode 130, the RV coil
electrode 132, and the SVC coil electrode 134, respectively.
At the core of the stimulation device 100 is a programmable
microcontroller 220 that controls the various modes of operation.
As is well known in the art, microcontroller 220 typically includes
a microprocessor, or equivalent control circuitry, designed
specifically for controlling the delivery of stimulation therapy,
and may further include RAM or ROM memory, logic and timing
circuitry, state machine circuitry, and I/O circuitry. Typically,
microcontroller 220 includes the ability to process or monitor
input signals (data or information) as controlled by a program code
stored in a designated block of memory. The type of microcontroller
is not critical to the described implementations. Rather, any
suitable microcontroller 220 may be used that carries out the
functions described herein. The use of microprocessor-based control
circuits for performing timing and data analysis functions are well
known in the art. As described herein, an implantable device
includes potential field generating and potential field sensing
capabilities, which are optionally controllable via a
microcontroller.
Representative types of control circuitry that may be used in
connection with various exemplary device and/or methods described
herein can include aspects of the microprocessor-based control
system of U.S. Pat. No. 4,940,052 (Mann et al.), the state-machine
of U.S. Pat. No. 4,712,555 (Thornander et al.) and U.S. Pat. No.
4,944,298 (Sholder), all of which are incorporated by reference
herein. For a more detailed description of the various timing
intervals used within a typical stimulation device and their
inter-relationship, see U.S. Pat. No. 4,788,980 (Mann et al.), also
incorporated herein by reference.
FIG. 2 also shows an atrial pulse generator 222 and a ventricular
pulse generator 224 that generate potential field and/or pacing
stimulation pulses for delivery by the right atrial lead 104, the
coronary sinus lead 106, and/or the right ventricular lead 108 via
an electrode configuration switch 226. It is understood that in
order to generate potential fields and/or to provide stimulation
therapy in each of the four chambers of the heart, the atrial and
ventricular pulse generators, 222 and 224, may include dedicated,
independent pulse generators, multiplexed pulse generators, or
shared pulse generators. The pulse generators 222 and 224 are
controlled by the microcontroller 220 via appropriate control
signals 228 and 230, respectively, to trigger or inhibit potential
field generation and/or stimulation pulses.
Microcontroller 220 further includes timing control circuitry 232
to control the timing of potential field generation, potential
sensing and/or stimulation pulses (e.g., pacing rate,
atrio-ventricular (AV) delay, atrial interconduction (A--A) delay,
or ventricular interconduction (V--V) delay, etc.) as well as to
keep track of the timing of refractory periods, blanking intervals,
noise detection windows, evoked response windows, alert intervals,
marker channel timing, etc.
Microcontroller 220 further includes an arrhythmia detector 234, a
morphology detector 236, and in one embodiment a minute ventilation
(MV) detection module 238 and CO2 level detection module 237. The
aforementioned components may be implemented in hardware as part of
the microcontroller 220, or as software/firmware instructions
programmed into the device and executed on the microcontroller 220
during certain modes of operation.
The electronic configuration switch 226 includes a plurality of
switches for connecting the desired electrodes to the appropriate
I/O circuits, thereby providing complete electrode programmability.
Accordingly, switch 226, in response to a control signal 242 from
the microcontroller 220, determines the polarity of potential field
generations, potential sensing and/or stimulation pulses (e.g.,
unipolar, bipolar, combipolar, etc.) by selectively closing the
appropriate combination of switches (not shown).
Atrial sensing circuits 244 and ventricular sensing circuits 246
may also be selectively coupled to the right atrial lead 104,
coronary sinus lead 106, and the right ventricular lead 108,
through the switch 226 for detecting the presence of cardiac
activity in each of the four chambers of the heart. In addition,
such circuits are optionally used to sense potentials, for example,
in a potential field. Accordingly, the atrial (ATR. SENSE) and
ventricular (VTR. SENSE) sensing circuits, 244 and 246, may include
dedicated sense amplifiers, multiplexed amplifiers, or shared
amplifiers. Switch 226 determines the "sensing polarity" of any
sensed signal by selectively closing the appropriate switches. In
this way, the clinician may program the sensing polarity
independent of potential field generation and/or stimulation
polarity. The sensing circuits (e.g., 244 and 246) are optionally
capable of obtaining information indicative of tissue capture.
Each sensing circuit 244 and 246 preferably employs one or more low
power, precision amplifiers with programmable gain and/or automatic
gain control, bandpass filtering, and a threshold detection
circuit, to selectively sense the signal (or potential) of
interest. The automatic gain control enables the device 100 to deal
effectively with the difficult problem of sensing low amplitude
signal characteristics associated with atrial or ventricular
fibrillation.
The outputs of the atrial and ventricular sensing circuits 244 and
246 are connected to the microcontroller 220, which, in turn, is
able to trigger or inhibit the pulse generators 222 and 224,
respectively, in a demand fashion in response to the absence or
presence of cardiac activity in the appropriate chambers of the
heart. Furthermore, as described herein, the microcontroller 220 is
also capable of analyzing information output from the sensing
circuits 244 and 246 and/or the data acquisition system 252 to
determine or detect whether and to what degree tissue capture has
occurred and to program a pulse, or pulses, in response to such
determinations. The sensing circuits 244 and 246, in turn, receive
control signals over signal lines 248 and 250 from the
microcontroller 220 for purposes of controlling the gain,
threshold, polarization charge removal circuitry (not shown), and
the timing of any blocking circuitry (not shown) coupled to the
inputs of the sensing circuits, 244 and 246.
For arrhythmia detection, the device 100 utilizes the atrial and
ventricular sensing circuits, 244 and 246, to sense cardiac signals
to determine whether a rhythm is physiologic or pathologic. The
timing intervals between sensed events (e.g., P-waves, R-waves, and
depolarization signals associated with fibrillation which are
sometimes referred to as "F-waves" or "Fib-waves") are then
classified by the arrhythmia detector 234 of the microcontroller
220 by comparing them to a predefined rate zone limit (i.e.,
bradycardia, normal, low rate VT, high rate VT, and fibrillation
rate zones) and various other characteristics (e.g., sudden onset,
stability, physiologic sensors, and morphology, etc.) in order to
determine the type of remedial therapy that is needed (e.g.,
bradycardia pacing, anti-tachycardia pacing, cardioversion shocks
or defibrillation shocks, collectively referred to as "tiered
therapy").
Cardiac signals and/or sensed potentials are also applied to inputs
of an analog-to-digital (A/D) data acquisition system 252. The data
acquisition system 252 is configured to acquire intracardiac
electrogram (IEGM) signals and/or potentials, convert the raw
analog data into a digital signal, and store the digital signals
for later processing and/or telemetric transmission to an external
device 254. The data acquisition system 252 is coupled to the right
atrial lead 104, the coronary sinus lead 106, and the right
ventricular lead 108 through the switch 226 to sample potentials
and/or cardiac signals across any pair of desired electrodes
(including can or case or other electrodes).
The microcontroller 220 is further coupled to a memory 260 by a
suitable data/address bus 262, wherein the programmable operating
parameters used by the microcontroller 220 are stored and modified,
as required, in order to customize the operation of the implantable
device 100 to suit the needs of a particular patient. Such
operating parameters define, for example, pacing pulse amplitude,
pulse duration, electrode polarity, rate, sensitivity, automatic
features, arrhythmia detection criteria, and the amplitude,
waveshape and vector of each shocking pulse to be delivered to the
patient's heart 102 within each respective tier of therapy. One
feature is the ability to sense and store a relatively large amount
of data (e.g., from the data acquisition system 252), which data
may then be used for subsequent analysis to guide the programming
of the device.
Advantageously, the operating parameters of the implantable device
100 may be non-invasively programmed into the memory 260 through a
telemetry circuit 264 in telemetric communication via communication
link 266 with external device 254, such as a programmer,
transtelephonic transceiver, or a diagnostic system analyzer. The
microcontroller 220 activates the telemetry circuit 264 with a
control signal 268. The telemetry circuit 264 advantageously allows
intracardiac electrograms (IEGMs) and status information relating
to the operation of the device 100 (as contained in the
microcontroller 220 or memory 260) to be sent to the external
device 254 through an established communication link 266.
The implantable device 100 can further include a physiologic sensor
270, commonly referred to as a "rate-responsive" sensor because it
is typically used to adjust pacing stimulation rate according to
the exercise state of the patient. However, the physiological
sensor 270 may further be used to detect changes in cardiac output,
changes in the physiological condition of the heart, or diurnal
changes in activity (e.g., detecting sleep and wake states).
Accordingly, the microcontroller 220 responds by adjusting the
various pacing parameters (such as rate, AV Delay, V--V Delay,
etc.) at which the atrial and ventricular pulse generators, 222 and
224, generate stimulation pulses.
While shown as being included within the implantable device 100, it
is to be understood that the physiologic sensor 270 may also be
external to the stimulation device 100, yet still be implanted
within or carried by the patient. Examples of physiologic sensors
that may be implemented in device 100 include known sensors that,
for example, sense respiration, blood pH, CO.sub.2 level, and so
forth. Another sensor that may be used is one that detects activity
variance, wherein an activity sensor is monitored diurnally to
detect the low variance in the measurement corresponding to the
sleep state. For a more detailed description of an activity
variance sensor, the reader is directed to U.S. Pat. No. 5,476,483
(Bornzin et al.) and U.S. Pat. No. 6,128,534 to Park et al., which
patents are hereby incorporated by reference. Each of these patents
is incorporated by reference herein. In one illustrative
embodiment, a pH (or CO.sub.2) sensor, respiration-related (MV)
sensor, and activity sensor are all included in the system, as is
described in more detail below.
In one embodiment, the physiological sensors 270 preferably include
sensors to help detect movement of the patient. The physiological
sensors 270 may include a position and/or activity sensor. Signals
generated by the position and/or activity sensor are passed to the
microcontroller 220 for analysis, as described in greater detail
below. The microcontroller 220 monitors the signals for indications
of the patient's position and activity status, such as whether the
patient is climbing upstairs or descending downstairs or whether
the patient has been lying down for an extended period of time,
thereby indicating a prolonged rest or sleep state.
The implantable device additionally includes a battery 276 that
provides operating power to all of the circuits shown in FIG. 2.
For the implantable device 100, which may employ shocking therapy,
the battery 276 is capable of operating at low current drains for
long periods of time (e.g., preferably less than 10 .mu.A), and is
capable of providing high-current pulses (for capacitor charging)
when the patient requires a shock pulse (e.g., preferably, in
excess of 2 A, at voltages above 2 V, for periods of 10 seconds or
more). The battery 276 also desirably has a predictable discharge
characteristic so that elective replacement time can be
detected.
The implantable device 100 can further include magnet detection
circuitry (not shown), coupled to the microcontroller 220, to
detect when a magnet is placed over the implantable device 100. A
magnet may be used by a clinician to perform various test functions
of the implantable device 100 and/or to signal the microcontroller
220 that the external programmer 254 is in place to receive or
transmit data to the microcontroller 220 through the telemetry
circuits 264.
The implantable device 100 further includes an impedance measuring
circuit 278 that is enabled by the microcontroller 220 via a
control signal 280. The impedance measuring circuit 278 measures an
impedance value, which can be used as a surrogate for respiration
(tidal volume or minute ventilation), as described in further
detail below. The impedance measuring circuit 278 is preferably
coupled to the switch 226 so that any desired electrode may be
used, preferably electrodes that measure transthoracic impedance.
Further aspects of impedance are described below, especially the
ability to measure respiration (e.g., tidal volume) in connection
with the illustrative embodiments below.
In the case where the implantable device 100 is intended to operate
as an implantable cardioverter/defibrillator (ICD) device, it
detects the occurrence of an arrhythmia, and automatically applies
an appropriate therapy to the heart aimed at terminating the
detected arrhythmia. To this end, the microcontroller 220 further
controls a shocking circuit 282 by way of a control signal 284. The
shocking circuit 282 generates shocking pulses of low (up to 0.5
J), moderate (0.5 J to 10 J), or high energy (11 J to 40 J), as
controlled by the microcontroller 220. Such shocking pulses are
applied to the patient's heart 102 through at least two shocking
electrodes, and as shown in this embodiment, selected from the left
atrial coil electrode 126, the RV coil electrode 132, and/or the
SVC coil electrode 134. As noted above, the housing 200 may act as
an active electrode in combination with the RV electrode 132, or as
part of a split electrical vector using the SVC coil electrode 134
or the left atrial coil electrode 126 (i.e., using the RV electrode
as a common electrode).
Cardioversion level shocks are generally considered to be of low to
moderate energy level (so as to minimize pain felt by the patient),
and/or synchronized with an R-wave and/or pertaining to the
treatment of tachycardia. Defibrillation shocks are generally of
moderate to high energy level (i.e., corresponding to thresholds in
the range of 5 J to 40 J), delivered asynchronously (since R-waves
may be too disorganized), and pertaining exclusively to the
treatment of fibrillation. Accordingly, the microcontroller 220 is
capable of controlling the synchronous or asynchronous delivery of
the shocking pulses.
Hypercapnic Ventilatory Response as Indicator of Change in
Physiologic Condition
Various exemplary diagnostics and therapies will now be described.
FIG. 3 shows an exemplary flow chart depicting one illustrative
embodiment of a method for tracking a progression or regression of
a physiologic condition, such as CHF, pulmonary congestion or
edema, or detecting a patient who is likely to suffer from CSR.
According to the illustrative method shown in FIG. 3, operation
commences at block 300, with device 100 measuring a
CO.sub.2-related parameter and a respiration-related parameter.
In one embodiment, a suitable sensor 75 (FIG. 1) is provided for
measuring the CO.sub.2-related parameter and preferably is, for
example, a blood gas (e.g., CO.sub.2) sensor, pH sensor, and the
like. Examples of sensors that can be modified for use in the
stimulation devices of the present invention are disclosed, for
example, in U.S. Pat. No. 4,816,131 (Bomsztyk), and U.S. Pat. No.
4,716,887 to Konig et al., which are incorporated herein by
reference. Suitable sensors and sensing techniques are well known
to one of skill in the art and can be readily adapted for use in
the present invention. Thus, in one embodiment the sensor 75 may
detect CO.sub.2 directly, or may be a pH sensor, and is connected
to controller 220 via terminal 221, from which the CO.sub.2 level
in the patient's blood can be inferred by CO.sub.2 detection module
237. In one illustrative embodiment, the sensor that measures the
CO.sub.2-related parameter is located on one of leads 104, 106, and
108, for example on the coronary sinus lead 106 (FIG. 1), to detect
blood in the passage that returns deoxygenated blood from the
capillaries of the heart; alternately, the CO.sub.2 sensor can be
located on the can 200, or on an additional lead (not shown)
located within the patient's body for contact with blood. As is
shown in FIG. 1, sensor 75 may be located on lead 106 and/or lead
108 (shown in dashed lines to represent potential alternate
location for sensor 75), or any other suitable location, for
example within the right atrium.
As described generally above, minute ventilation (also referred to
as "minute volume" or "MV") is a respiratory-related parameter that
is a measure of the volume of air inhaled and exhaled during a
particular period of time. A minute ventilation signal can be
obtained by measuring transthoracic (across the chest or thorax)
impedance. Transthoracic impedance provides respiratory or
ventilation information, including how fast and how deeply a
patient is breathing. A component of transthoracic impedance varies
as the patient inhales and exhales. Ventilation (e.g., breathing
rate, which is also referred to as "ventilation rate" or "VR", and
breathing volume, which is also referred to as "tidal volume" or
"TV") information is included in the impedance signal, and is
preferably used in the disclosed embodiments as described below. As
is well known to those skilled in the art, the magnitude of the
change of the impedance signal corresponds to the tidal volume and
the frequency of change corresponds to respiration rate. Thus, by
monitoring the amplitude of the impedance signal, the tidal volume
value can be readily obtained.
A minute ventilation signal is derived from the impedance signal,
as illustrated by Equation 1. MV measures air flow rate (e.g.,
liters per minute), TV measures volume per breath (e.g., liters per
breath), and VR measures breathing rate (e.g., breaths per minute),
as shown in the following equation: MV=TV.times.VR (1)
By way of example, approaches for measuring transthoracic impedance
are described in Hauck et al., U.S. Pat. No. 5,318,597 entitled
"RATE ADAPTIVE CARDIAC RHYTHM MANAGEMENT DEVICE CONTROL ALGORITHM
USING TRANS-THORACIC VENTILATION," assigned to the assignee of the
present application, the disclosure of which is incorporated herein
by reference; in U.S. Pat. No. 4,816,131 to Bomsztyk, and in U.S.
Pat. No. 5,836,988 to Cooper et al., which are all incorporated
herein by reference.
Referring again to FIG. 3, once the CO.sub.2-related parameter and
respiration-related parameter have been obtained, operation
proceeds to block 302, and the CO.sub.2-related parameter is
compared with the respiration-related parameter. In one
illustrative embodiment, a ratio of the CO.sub.2-related value to
the respiration-related value is computed, although other suitable
comparisons may be used, which will be apparent to those skilled in
the art. For example, the ratio of CO.sub.2/TV may be used.
At query block 304, system 100 determines whether the comparison
changes over time. In one embodiment, the ratio of CO.sub.2/TV is
periodically computed, e.g., daily, weekly, monthly, and the like.
If, for example, the ratio increases over time, if it exceeds a
preset threshold value, changes by more than a certain percent from
an initial value, or changes by more than a certain percent from
the previous value, then operation proceeds to block 306. If not,
operation returns to block 300 to take the next measurement, which
could be daily, weekly, monthly, or any other suitable interval
between measurements.
If the comparison is positive at query block 304, operation
proceeds to block 306, and the system 100 takes appropriate action.
In one embodiment, system 100 may alert the patient through any
well-known alert mechanism, thereby alerting the patient to seek
medical help. Alternatively, system 100 may telemeter an alert via
telemetry circuit 264 to external device 254, which may then
transmit the alert transtelephonically, over a computer network, or
the like, to the patient's physician. Moreover, in response to a
positive result, the system 100 may change one or more operating
parameters of the device, for example, implementing bi-ventricular
pacing. In addition, system 100 may simply store the data and a
suitable message for transfer to the physician during a subsequent
follow-up interrogation of system 100.
FIG. 4 shows an exemplary flow diagram for calculating tidal volume
values. At block 400, the peaks and valley of the tidal volume
signal (as derived from the impedance signal) are measured. At
block 402, the tidal volume is calculated as the difference between
the peak and valley values (although other suitable measurements
could be used as well, such as the magnitude of a rectified signal
and the like). At block 404, tidal volume values are obtained for a
plurality of breaths, to determine breath-by-breath tidal volume
values, which are then used with corresponding CO.sub.2 values to
determine the CO.sub.2/TV ratio values as described above.
FIG. 5 shows a flow diagram of another illustrative embodiment for
detecting progression of a physiologic condition. At query block
500, system 100 determines whether the patient is asleep or in a
prolonged resting state. Preferably, this is determined by use of
the activity sensor, for example by detecting low activity coupled
with low activity variance, as described in the Bornzin patent
cited above.
If the patient is asleep or in a prolonged rest state, operation
proceeds to block 502, and system 100 detects both the CO.sub.2
value and respiration-related value, preferably as described in
detail above. Moreover, the ratio of CO.sub.2/respiration-related
value is computed. Operation proceeds to block 504, and the ratio
is preferably averaged over at least several cycles. The value is
then stored in memory.
At block 506, multiple ratio values, taken over an extended period
of time (e.g., a day, a week, a month, etc.), are analyzed for any
upward or downward trend in the data. For example, if the ratio
increases by more than a preset percentage from the initial value
(or from the previous value), or if the ratio exceeds a threshold
value, then a potential change in a physiologic condition is
indicated, such as progression of heart failure, pulmonary
congestion, or an increased likelihood that a patient suffers from
CSR. Thus, at query block 508, if the ratio increases overtime,
operation proceeds to block 510 and appropriate action is taken. As
described above, such action could be alerting the patient,
alerting the patient's physician, adjusting one or more operating
parameters of system 100, storing corresponding data to be
retrieved during the next interrogation, and the like. It will also
be apparent to those skilled in the art that if the ratio decreases
over time, action can be taken, such as adjusting operating
parameters, alerting the physician, and the like.
FIG. 6 shows an exemplary flow diagram of another illustrative
embodiment. Operation begins at block 600, with system 100
measuring the CO.sub.2-related parameter and respiration-related
parameter as described in detail above. At block 602, system 100
compares the CO.sub.2-related parameter with the
respiration-related parameter over time. At query block 604, if the
comparison changes over time (e.g., if a ratio increases by a
preset percentage, or exceeds a threshold, etc.), operation
proceeds to block 606, and system 100 detects a change in a
physiologic condition, such as an increase in pulmonary congestion,
which could lead to pulmonary edema. Preferably, system 100 will
alert either the patient or physician, or both.
CONCLUSION
Although various exemplary devices and/or methods have been
described in language specific to structural features and/or
methodological acts, it is to be understood that the subject matter
defined in the appended claims is not necessarily limited to the
specific features or acts described. Rather, the specific features
and acts are disclosed as exemplary forms of implementing the
claimed subject matter.
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